In the third of his series on global food markets, BBC World Affairs Correspondent Mark Doyle gets into a 'Bod Pod' to investigate how modern eating habits are making many of us fat.

The 'Bod Pod' measures the air displaced inside the chamber. Photo: Pat Dalton, University of Ulster

I regretted suggesting a visit to an obesity clinic almost as soon as the words had left my lips.

But my colleague, BBC producer Ed Butler, immediately seized upon it as an excellent way of illustrating the increasingly important issue of global obesity.

So I found myself at Addenbrooke's Hospital, in the east of England, in the company of Dr Nick Finer, a senior medical consultant on obesity.

Dr Finer told me to take my shoes off and clamber inside a sort of sealed air chamber called a 'Bod Pod'.

"You just climb in there, Mark, and I'll be able to measure how much of your body is fat."

I climbed in; the machine started whirring away.

More people in the world are now overweight than underweight

It was around eight years ago that the number of overweight people in the world began exceeding the number of underweight people - many of them seriously undernourished.

The figures switched because, contrary to popular prejudice, it's not just Americans and Belgians who now tend to be fat.

Citizens of the developing world - from Mexico, for example, Egypt and South Africa - are also tipping the scales as overweight.

Poverty reduced

"Since the turn of the century," said Barry Popkin, Professor of Nutrition at the University of North Carolina, "we've seen big reductions in poverty, particularly in India and China."

"So we've had large reductions in the number of people who are underweight. At the same time we've had far faster increases in the numbers of those overweight."

....between 1.3 billion and 1.6 billion are overweight

Professor Barry Popkin, University of North Carolina

"In every country in the world, the rate of increase in the number becoming overweight and obese has accelerated."

"So today we have reached a point where we have, depending on the estimates, between 1.3bn and 1.6bn overweight people in the world, but fewer, between 700m and 800m, who are underweight."

The reasons why more people than ever before are putting on weight are, on the face of it, simple enough; they eat too many fatty or sweet foods, and don't do enough exercise.

Least healthy

But behind that truism are several complex processes.

Barry Popkin says the way we have organised agriculture is a key factor:

"Over the past century we have worked very hard to produce cheap beef. The global price of beef has gone down to about a quarter of its cost in the 1950s.

Millions more around the globe can now afford to eat beef

"This makes it that much easier for poor people to consume beef, and the same has occurred for edible oils and sugars."

"These products are the least healthy in terms of producing diabetes, cancer and obesity."

In contrast to meat, oils and sugars, Professor Popkin concludes, "We have in global budgetary terms probably invested one thousandth of 1% in encouraging the consumption of fruit and vegetables."

Back at the controls of 'Bod Pod', Dr Nick Finer was doing calculations about my body mass. Since I know I am a little overweight I wasn't overly keen to get the results.

So I tried to distract him with some questions about why we often eat food that is not good for us.

Inappropriate messages

"In the past, when we were hunter-gatherers, our bodies were programmed to eat what we could when we could," said Dr Finer. "We needed to store food because we didn't know where the next meal would come from."

But now that agriculture has delivered plenty, the messages our brains give to our bodies may no longer be appropriate.

"The human brain is very good at telling us we are hungry and need food. But the problem now is that there is always plenty available and the brain is not so good at telling us we have had enough."